The present invention relates to linear copolymers of fluorocarbon-hydrocarbon monomers having desirable attributes of both monomers, and linear co- and terpolymers thereof, and a method of synthesis in liquid or supercritical carbon dioxide.
Fluoropolymers have been used in a wide array of applications, including those in coatings (Jones, C. W. U.S. Pat. No. 5,723,556 1998), optical (Feiring, A. E.; Wonchoba, E. R. Macromolecules 1998 31, 7103-7104), and biomedical fields (Kogel, H.; Vollmar, J. F.; Proscheck, M. B.; Scharf, G.; Buftel, H. M. in Prosthetic Substitution of Blood Vessels, H. Kogel, Ed.; Quintessen-Verlags: Munchen, Germany, 1991, p. 143). However, the inherent thermal stability of commercially available fluoropolymers limits their processability while the inherent chemical stability limits their solubility in organic solvents. One approach to prepare a processable, organic solvent-soluble, functionalized fluoropolymer, is to copolymerize a fluorocarbon monomer, such as tetrafluoroethylene (TFE), with a hydrocarbon monomer, such as vinyl acetate (VAc).
Modena et al. (Modena, M.; Borsini, G.; Ragazzini, M.; European Polymer J. 1967 3, 5-12), U.S. Pat. Nos. 5,032,656 (Mares et al.) and 5,723,556 (Jones, C. W.) disclose copolymers of fluoromonomers, such as tetrafluoroethylene (TFE) and chlorotrifluoroethylene (CTFE), and functionalized hydrocarbon monomers, such as vinyl acetate (VAc). The copolymers were prepared by free radical polymerization in aqueous emulsions that may employ an organic co-solvent. The copolymers were amorphous, organic solvent soluble, and could be cast into colorless transparent films. Partial or complete hydrolysis of copolymers incorporating VAc, to vinyl alcohol (VA), yielded terpolymers and copolymers, respectively, with reactive functional sites for potential further modification, such as cross-linking. However, the solvent systems used for polymerization limited these copolymers. A narrow range of compositions was obtained when organic co-solvents were employed (U.S. Pat. No. 5,032,656) while highly branched structures were formed, for copolymers prepared in aqueous emulsions (U.S. Pat. No. 5,723,556).
Hydrolysis of the latter copolymers, prepared in aqueous emulsions, not only converted ester groups to the corresponding vinyl alcohol but also resulted in at least a 10-fold decrease in the weight average molar mass (Mw) of the copolymers. Furthermore, there was also a large decrease in intrinsic viscosity with hydrolysis (Modena et al. European Polymer Journal 1967). It was proposed that radical hydrogen abstraction, of methyl hydrogen from VAc by the macro-radical on TFE, and continued propagation of the resulting macro-radical, incorporated ester groups into the polymer backbone (U.S. Pat. No. 5,723,556). Subsequent hydrolysis cleaved these esters resulting in a decreased Mw.
DeSimone, J. M.; Guan, Z.; Elsbernd, C. S. Science 1992 257, 945-947, have shown that liquid or supercritical carbon dioxide (CO2) is an environmentally friendly and convenient solvent for fluoropolymer synthesis. U.S. Pat. No. 5,618,894 (DeSimone, J. M. and Romack, T.) discloses a process for preparing homo- and copolymers of fluorocarbon monomers, and copolymers of fluorocarbon monomers with hydrocarbon monomers, such as ethylene in CO2, but does not disclose linear copolymers. Furthermore, CO2 was used for the dispersion homo- and copolymerization of VAc with other hydrocarbon monomers. Relatively high molar mass fluorocarbon-block-hydrocarbon copolymer surfactants were used to stabilize the polymer colloid as it formed (see Canelas, D. A.; Betts, D. E.; DeSimone, J. M.; Yates, M. Z.; Johnston, K. P. Macromolecules 1998 31, 6794-6805) since most polymers have limited or negligible solubility in CO2. Exceptions include amorphous fluoropolymers and some poly(siloxanes) (DeSimone et al. Chem. Rev. 1999).
Surfactants may be used for a dispersion polymerization and are used for an emulsion polymerization in CO2. In a dispersion polymerization, all components are initially soluble in the continuous CO2 phase, whereas in an emulsion polymerization, both monomer and polymer are insoluble in the continuous phase. As the growing polymer chains in a dispersion polymerization reach a critical molar mass, the chains become insoluble and phase separate. At this point, surfactant molecules absorb to the colloid particle surface and prevent coagulation. Polymerization continues in both the continuous phase and the colloid polymer particles. The high molar mass, fluorocarbon-block-hydrocarbon copolymer surfactants and a low molar mass, commercially available, fluorocarbon-block-hydrocarbon oligomeric copolymer (Zonyl FSO-100(trademark)) were shown to form micelles in CO2 (Fulton et al., DeSimone et al. Langmuir 1995). However, Zonyl FSO-100(trademark) and similar materials, such as Fluorad(trademark) FC-171 or FC-170C, have not previously been used as surfactants for polymerization in CO2. Given that most small molecules have good solubility in CO2, dispersion polymerizations constitute the majority of examples in the literature. However surfactants are not required for successful polymerizations in CO2 of fluorocarbon-hydrocarbon copolymers.
DeSimone et al. Polymer Preprints 1997 also copolymerized TFE with perfluoropropylvinyl ether (PPVE) in CO2. As a consequence of copolymerization in CO2, the concentration of acid end-groups, resulting from undesirable xcex2-scission chain transfer of PPVE, were at least 365 times lower than copolymers prepared in conventional solvents. The authors concluded that this was a consequence of propagation competing more effectively with xcex2-scission resulting from CO2""s proven invasive ability to facilitate diffusion of small molecules. This has been shown to be true, even of small molecules diffusing into crystalline fluoropolymer matrices (McCarthy et al. Macromolecules 1995). The rate of propagation, a bimolecular process, increased in the polymer phase. Since xcex2-scission is a unimolecular process, changes in diffusion had little effect on its rate, and propagation relative to xcex2-scission was more favorable in CO2 than in conventional solvents.
It would be very advantageous to provide predominantly linear copolymers comprising fluorocarbon monomers and hydrocarbon monomers.
It is an object of the present invention to provide linear fluoropolymers, which are copolymers of fluorocarbon and hydrocarbon repeat units.
In one aspect of the invention there is provided a linear copolymer comprising fluorocarbon and hydrocarbon repeat units, with the fluorocarbon repeat unit being selected from the group consisting of tetrafluoroethylene, trifluoroethylene, chlorotrifluoroethylene, vinylidene fluoride, and vinyl fluoride and said hydrocarbon repeat unit being selected from the group consisting of vinyl acetate, vinyl alcohol, acrylic acid, acrylates and methacrylates.
The copolymers may contain a broad range of the fluorocarbon repeat units and are essentially linear. That is, there is a minimal decrease in molar mass and typically no decrease in viscosity upon hydrolysis, indicative of either no, or very low concentrations, of branches (such as esters) in the polymer backbone.
In another aspect of the present invention there is provided a linear copolymer comprising fluorocarbon monomers and hydrocarbon monomers, said fluorocarbon monomers having a formula CF2xe2x95x90CR1R2, wherein C is carbon, F is fluorine, R1 is one of hydrogen, halogen, alkyl, aryl, haloalkyl, alkoxy, haloalkoxy, haloaryl and wherein R2 is one of hydrogen and halogen, said hydrocarbon monomers having a formula CH2xe2x95x90CR3CO2R4, wherein R3 is one of hydrogen and an alkyl, and R4 is one of hydrogen, an alkyl group and a haloalkyl group.
In a further aspect of the present invention there is provided a linear copolymer comprising fluorocarbon monomers and hydrocarbon monomers, fluorocarbon monomers having a formula, CF2xe2x95x90CR1R2, wherein C is carbon, R1 is selected from the group consisting of hydrogen, halogen, alkyl, aryl, haloalkyl, alkoxy, haloalkoxy, haloaryl and R2 is selected from the group consisting of hydrogen, halogen, said hydrocarbon monomers having a formula, CH2xe2x95x90CR3OC(O)R4 wherein R3 is one of hydrogen and alkyl, and R4 is an alkyl group.
In these two aspects of the invention linear copolymers or terpolymers may be produced by contacting the previously mentioned linear copolymers with a hydrolyzing or trans-esterification agent. Copolymers which contain a hydrocarbon monomer of the formula CH2xe2x95x90CR3OC(O)R4, wherein R3 is H or alkyl and R4 is an alkyl group that may be partially or almost completely hydrolyzed to the vinyl alcohol, yielding terpolymers and copolymers, respectively. Copolymers which contain a hydrocarbon monomer of the formula CH2xe2x95x90CR3CO2R4, wherein R3 is H or alkyl, and R4 is an alkyl group may be trans-esterified to ester groups, CO2R5, where R5 is a different group such as alkyl or haloalkyl. Additionally, R4 may be hydrolyzed to the carboxylic acid or salt.
The copolymer may be poly(tetrafluoroethylene-co-vinyl acetate) and hydrolysis of the VAc thereof, poly(chlorotrifluoroethylene-co-vinyl acetate) and hydrolysis of the VAc thereof, poly(vinylidene fluoride-co-vinyl acetate) and hydrolysis of the VAc thereof, poly(tetrafluoroethylene-co-acrylic acid) and esterification of the AA thereof, poly(tetrafluoroethylene-co-tertiary butyl acrylate) and transesterification of the tBA thereof.
The present invention also provides a method of synthesizing a linear copolymer comprising fluorocarbon and hydrocarbon repeat units, the method comprising mixing a fluorocarbon monomer, an initiator, an oligomeric nonionic surfactant, a hydrocarbon monomer and a polymerization medium comprising supercritical carbon dioxide and copolymerizing said fluorcarbon monomer and said hydrocarbon monomer.
In this aspect of the invention the mixture may contain an oligomeric nonionic surfactant having a CO2 phobic portion and a CO2 philic portion.
This invention provides a method of synthesizing linear terpolymers or copolymers, comprising;
providing a linear copolymer comprising fluorocarbon and hydrocarbon monomers;
hydrolyzing said linear copolymer in an effective acid medium; and
collecting a linear copolymer or terpolymer produced by said acid hydrolysis.
There is also provided a method of synthesizing linear terpolymers or copolymers, comprising:
providing a linear copolymer comprising fluorocarbon and hydrocarbon monomers;
esterifying said linear copolymer in an effective acidic medium; and collecting a linear copolymer or terpolymer produced by said esterification.
There is also provided a method of synthesizing linear terpolymers or copolymers, comprising:
providing a linear copolymer comprising fluorocarbon and hydrocarbon monomers;
trans-esterifying said linear copolymer in an effective acidic medium; and collecting a linear copolymer or terpolymer produced by said trans-esterification.